Photoreception and sensory transduction will be investigated for two blue- light responses of the unicellular sporangiophore of the fungus Phycomyces. Both phototropism and the light-growth response occur over an absolute operating range of 10 decades in light intensity from 10-9 to 10 W m-2. This range and the associated adaptation phenomena are similar to those that occur in visual receptor cells. The long term objective of the proposed research is to understand the processes by which a light stimulus is transduced into a cellular response, particularly in organisms with specific blue-light sensitivity associated with flavin chromophores. In Phycomyces, a model organism, genetic and physiological approaches can be effectively combined. Action spectra for phototropism and the light-growth response will be measured in different intensity ranges to characterize the low and high intensity photosystems of the sporangiophore. Phototropism action spectra will be obtained with continuous and pulsed light protocols. Experiments on kinetics of phototropism will be recorded with time-lapse video equipment and analyzed with the aid of an tracking machine using a novel method. The results will be compared with phototropic balance spectra to determine how each of the blue-light responses uses the components of the photoreceptor system. The recently discovered dependence of dark adaptation on dim, subliminal light will be pursued to help establish which photoreceptor mediates this effect, which is sensitive to green and red as well as blue light. System-identification experiments on the light-growth response will be continued on the tracking machine with Gaussian white noise and sum-of-sinusoids test stimuli to measure the dynamic and nonlinear aspects of this response. These experiments will involve both wild-type and mutant strains, and will be interpreted with analytical models for the kinetics of the photosensory transduction chain. A new theory, developed elsewhere, concerning light distribution around a sporangiophore will be tested experimentally by recording the azimuthal light pattern around a sporangiophore illuminated with a horizontal laser beam. This theory is important for interpretation of phototropism action spectra and is fundamental for theories of phototropism. Finally, cellular physiology methods will be applied to sporangiophores and protoplasts. Effector substances and indicator dyes will be introduced by direct uptake, injection, and electroporation; modified blue-light responses and effects will be monitored in preparations from wild type and mutant strains. These approaches are directed towards identifying molecular components and pathways associated with the blue light responses of Phycomyces.